Radiocarbon 14C

Background14C is a radioactive isotope of carbon. It was discovered
in 1934 by Grosse as an unknown activity in the mineral endialyte. In
the
same year, Kurie (Yale) exposed nitrogen to fast neutrons and observed
long tracks in a bubble chamber. He had produced 14C. It
was
in atmospheric CO2 by Libby in 1946. He determined the half
life to be 5568 years. This half life has later been re-determined by
Godwin.
The new half life is 5730 years. Libby recognized that due to its
occurrence
in natural materials, 14C can be used as a dating tool for
materials
that contain carbon compounds derived from atmospheric CO2
either
by simple mixing processes or by carbon exchange. The mean life time of
roughly 8000 years is ideal for dating of reservoirs that are a few
decades
to a few ten thousand yeas old. For groundwater, this means that 14C
dating can be applied to aquifers that contain water formed during
periods
that reach well into the past glacial time. 14C is a widely
used tool to establish chronologies for groundwater flow systems and
climate
records for the Holocene and Pleistocene. It is considered to be the
most
important tool for age dating of ‘old’ groundwater.

The challenge in 14C dating of groundwater is the
determination
of the initial 14C content of groundwater at the time of
recharge,
i.e., at the time when groundwater is isolated from exchange with the
soil
air and moves away from the water table.

There is also a stable isotope of carbon, 13C. This
isotope
is important in that it allows us to correct for carbon isotope
fractionation
in nature and during analytical procedures.

Natural 14C production

14C is mainly produced by interaction of cosmic ray derived
secondary neutrons with 14N in the atmosphere.

14N (n,p) 14C

14C can also be produced by the following reaction:

13C(d,p)14C d: deuterium or 2H

the production rate is 2.4 ± 0.2 atoms (cm2 sec)-1

Radioactive decay

14C decays by b-
decay
with a maximum energy of 0.158 MeV.

146C -> 147N+ e- +
anti-neutrino+ Q

Its half life t is 5730 years, i.e.,
somewhat
larger than the half life determined by Libby (5568 ys).

Natural global inventory

The global inventory of natural 14C is about 75 tons. The
specific
activity is » 13.56 dpm (gC)-1. dpm stands for decay
per
minute.

Anthropogenic 14C production

The main source of anthropogenic 14C is so-called ‘bomb’
14C,
i.e., 14C produced during atmospheric testing of nuclear
weapons.
At the peak of surface testing of nuclear devices in 1963, the
atmospheric
14C
activity had reached about twice that of natural 14C (Fig.
8.5
in Clark/Fritz). The bomb 14C has been produced by
interaction
of atmospheric nitrogen with the high neutron flux from the explosion
of
nuclear devices (mainly thermonuclear devices). Local increases in
atmospheric
14C
have been observed in the vicinity of nuclear power plants.

Notation

The notation of 14C activities is discussed in detail in
Stuiver
and Pollach (Radiocarbon, 19, 355-365, 1977). In short, 14C
is calibrated against an NBS (National Bureau of Standards) oxalic acid
standard. The internationally accepted radiocarbon dating reference is
95% of the activity, in 1950 AD, of the NBS oxalic acid normalized to d13C
of –13‰ with respect to PDB. The factor of 0.95 adjusts the oxalic acid
to the activity of wood from 1840 to 1860 (‘pre-industrial’).

A0N = 0.95 AOX [1 – 2(d13C
+ 19)/1000]

AON: 14C activity of oxalic acid normalized for
14C
fractionation

AOX: 14C activity of oxalic acid

The d13C correction of 19‰ takes
into account the fractionation of 14C during the combustion
of oxalic acid.

Groundwater:
For groundwater studies, the pmc (percent modern carbon) notation is
used.

pmc = (ASN/Aabs) 100% = ASN [AON
el(y-1950)]-1
100%

ASN: activity of sample normalized for fractionation using d13C

AON: 13C normalized activity of oxalic acid

Aabs: Absolute 14C activity of the sample

y: year of measurement of oxalic acid

14C dating

Principle:

In the atmosphere, 14C is incorporated into 14CO2
and takes part in the global carbon cycle. It is assimilated by plants.
Except for isotope fractionation, 14C in living organic
matter
is the same as that in atmospheric CO2. After organic matter
dies, the 14C concentration decreases due to radioactive
decay.
If there is no isotope exchange, radioactive decay is the only 14C
sink and if the initial 14C activity is known, an age can
be
calculated from the measured 14C activity of a sample.
In groundwater applications typically DIC (dissolved inorganic carbon;
DIC = CO2(aqueous) + HCO3- + CO32-)
is extracted from the water a and its measured 14C activity
is compared to the initial 14C activity. Determination of
the
initial 14C activity can be challenging and typically
requires
correction models that account for the carbon chemistry in the
unsaturated
and saturated soil zones.

The age equation derived above assumes a known initial 14C activity
of the sample. For natural atmospheric 14C, variability in
the
14C
production has to be reconstructed from calibrated tree ring
chronologies
or from coral records dated by U/Th. (Fig. 8.4; Clark and Fritz).

Initial 14C activity in groundwater

See summary in Clark and Fritz (chapter 8) for details

In short, the 14C activity of DIC (dissolved inorganic
carbon)
in groundwater is determined by the following factors: (Fig. 8.6 in
Clark
and Fritz):

dissolution of carbonates can occur in the unsaturated soil zone (open
system) or in the saturated soil zone (closed system).

isotope exchange can lead to a decrease in 14C activity in
addition
to radioactive decay

a variety of models can be used to estimate the initial 14C
activity in groundwater. They include the very simple ‘Vogel’ model,
several
models that correct for carbon chemistry using chemical or 13C
balances, and the complex NETPATH model by Plummer. The latter accounts
for the chemical evolution of the groundwater along flowpaths.

14CO2 + H2O + CaCO3
--> Ca2+ + HCO3-

completely open system: 14C activity: » 100 pmc

completely closed system: 14C activity: » 50 pmc

‘real world’ systems are somewhere in between open and closed and
the
correction models mentioned above and described in Clark and Fritz
(chapter
8) have to be applied.

Atmospheric concentrations

Determined by

Production in the atmosphere and its variability (Fig)
Bomb radiocarbon (Fig)
Suess effect (Fig)
Hemispheric and inter-hemispheric mixing (Fig.)
Local sources (Fig.)
Radioactive decay (minor effect; most of the 14C decays in the ocean)

Determined by measurements

Typically well known in clean air (Fig.)
Small hemispheric gradients (Fig.)
Inter-hemispheric gradients are of the order of xxxx percent

Measurement precision: ± 0.2 to ± 1% (1-s
error) for concentrations close to 100 pmc. This corresponds to an age
resolution of about ± 16 to ± 40 years.

Detection limit: <1% modern corresponding to ages of about 40000
years.

There are many laboratories worldwide that can measure 14C
routinely.
However, only few 14C laboratories have the capability to
measure
14C
at precisions of ± 0.2%. In groundwater studies the overall
error
is dominated by systematic errors and the analytical precision is not
as
critical as in oceanographic studies.